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Surface treatment of self tapping screw

2020-11-09



Surface treatment of self tapping screws: most self tapping screws are coated with zinc or zinc phosphate. Due to its high cost and toxicity, cadmium electroplating is rarely used. If appearance is important, nickel or chromium plating can be selected.

One is that surface treatment will not only increase scaling (such as reducing pores), but also affect the relationship between torque, tightening degree and strength. Cadmium plating or phosphate treatment has different rules. When testing the applicable tapping diameter, the screws used must be identical in appearance.


The second reason is that due to hydrogen embrittlement and high hardness, small size self tapping screws are often unavailable. Self tapping screws must be carburized to penetrate the iron plate. In this case, high hardness and high carbon content will occur after carburizing, and hydrogen embrittlement will occur during electroplating or pickling. The method of eliminating hydrogen embrittlement advocated in the code is: a). Bake within 1 hour after plating or pickling. b)。 The temperature is 375-425 of (190-220 OC). At least 4 hours.



Hydrogen embrittlement test: hydrogen embrittlement test is mainly to test whether there is residual hydrogen in the screw structure of self tapping screw, and whether it will make self tapping screw brittle. (embrittlement usually does not occur immediately, but must be given (stress is only shown). Plated or coated screws shall be installed in a test plate and steel flat washer as specified in Table 3, and standard plain washers shall be used on the lower bearing surface of a good head screw head. (oval) screws are made of matched chamfered spacer plates. The thickness of the flat washer or spacer installed on the lower bearing surface of the head shall be suitable for the maximum invalid tooth length of the screw.

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